Britain will be forced to build three cities the size of Birmingham within five years if it is to cope with current levels of immigration, an expert has warned.

It follows fears that he population could increase by three million by 2020 – adding strain to the country’s creaking public services,

The warning was issued by Lord Green of Deddington, chairman of think-tank MigrationWatch UK.

It will add pressure on Prime Minister David Cameron to take control of the borders as part of his renegotiation of Britain’s membership of the EU.

Immigration now tops the list of voters’ concerns in recent polls – with nearly six in 10 saying tackling immigration is among the most crucial issues facing the UK.

Lord Green said that if net migration – the number of people coming into the country minus those leaving – and the natural population increase continued at today’s rates, it would mean a typical rise of around 600,000 a year.

Giving evidence to the Home Affairs Select Committee, he said: ‘On current levels of net migration, we are going to have to build three cities the size of Birmingham within five years. That just can’t be done.’

Birmingham is the UK’s second largest city, has a population of just over one million, according to the 2011 census.

Lord Green said it would be ‘very difficult’ for the Government to hit its 100,000 migration target but he opposed axing the figure, arguing that it ‘focused’ bureaucrats and public opinion.

It comes just a week after Home Secretary Theresa May gave a stark speech to the Conservative Party conference warning that mass immigration made it impossible to build a cohesive society.